Sodium carbonate (Na2CO3), an environmentally friendly activator, has been shown to have vast potential for the development of sustainable alkali-activated slag mortars. However, Na2CO3-activated slag mortars exhibit a delayed reaction process and limited early-age strength development, restricting their wider application. In this work, the recycled concrete fines were calcined at a temperature of 800 °C for 1 h and then used as an auxiliary activator to improve the reaction kinetics of Na2CO3-activated slag mortars. The impact of the calcined recycled concrete fines (CRCF) dosage and Na2CO3 concentration on the compressive strength, hydration kinetics, and phase assemblage of mortars was evaluated. The results show that CRCF can react directly with Na2CO3 in the early stages, swiftly removing the CO32- in aqueous solution and providing an alkaline environment suitable for the dissolution of slag. This promotes the development of C-(A)-S-H, hydrotalcite, hemicarbonate, and monocarbonate. The hydration process and strength-giving phase of mortars can be improved further, as an increase in Na2CO3 concentration increases the initial alkaline content. Additionally, the most remarkable compressive strength value of 39.2 MPa was observed at 28 days in the mortar with 6% sodium oxide equivalent (Na2O-E) of Na2CO3 and 15% CRCF because of the synergistic effect of CRCF and Na2CO3.
Keywords: alkali-activated slag; calcined recycled concrete fines; compressive strength; hydration kinetics; sodium carbonate.